1. Field of the Invention
The present invention relates to a brake system having a master cylinder for generating brake fluid pressure based on a brake operation of a driver, and an electrically controlled fluid pressure generator (herein after “electrical fluid pressure generator”) for generating brake fluid pressure by an electrically controllable actuator.
2. Description of the Related Art Japanese Patent Application Laid-open No. 2003-137084 discloses a brake system of the type referred to as a brake by wire (BBW) brake system, which converts a brake operation of a driver into an electrical signal used to operate an electrical fluid pressure generator, and operates a wheel cylinder by brake fluid pressure generated by the electrical fluid pressure generator.
In this type of BBW brake system, if/when the electrical fluid pressure generator fails, brake fluid pressure generated by a master cylinder is directly transmitted to a wheel cylinder in order to brake the wheel, thereby exerting a failsafe function. Also, the master cylinder is of a tandem type in which the brake fluid pressure is supplied separately to two fluid pressure systems, so that even if a leakage occurs in one of the fluid pressure systems, the other fluid pressure system can brake any of the wheels controlled by the BBW brake system.
FIG. 7 shows a relevant portion of the structure of a conventional electrical fluid pressure generator (motor cylinder 123), excluding the motor and actuator. A rear piston 138A and a front piston 138B are slidably arranged within a cylinder main body 136, and urged by a pair of return springs 137A and 137B in a retreating direction. A rear fluid chamber 139A is defined in front of the rear piston 138A. A front fluid chamber 139B is defined in front of the front piston 138B.
Formed around an outer periphery of the rear piston 138A is a rear reservoir chamber 138a for preventing intrusion of air into the rear fluid chamber 139A. Formed around an outer periphery of the front piston 138B is a front reservoir chamber 138b for preventing intrusion of air into the front fluid chamber 139B. A rear inlet port 140A of the rear fluid chamber 139A and a rear supply port 149A of the rear reservoir chamber 138a communicate with the master cylinder. A rear outlet port 141A of the rear fluid chamber 139A communicates with a wheel cylinder. A front inlet port 140B of the front fluid chamber 139B and a front supply port 149B of the front reservoir chamber 138b communicate with the master cylinder. A front outlet port 141B of the front fluid chamber 139B communicates with the wheel cylinder.
A rear first cup seal C1′ is provided at a front end of the rear piston 138A so as to face forward (so that the seal function is exerted in moving forward). A rear second cup seal C2′ is provided at a rear end of the rear piston 138A so as to face forward. A front first cup seal C3′ is provided at a front end of the front piston 138B so as to face forward. A front second cup seal C4′ is provided at a rear end of the front piston 138B so as to face rearward (so that the seal function is exerted in moving rearward).
In the structure of this conventional motor cylinder 123, because the front supply port 149B is connected to the master cylinder, when the motor cylinder 123 fails and the wheel cylinder is operated by brake fluid pressure generated by the master cylinder, if the first fluid pressure system fails and the rear fluid chamber 139A of the motor cylinder 123 is opened to the atmosphere, the brake fluid pressure generated by the master cylinder leaks into a route through the front supply port 149B, the front reservoir chamber 138b, the front second cup seal C4′, and the rear fluid chamber 139A. Therefore, there is a possibility that the second fluid pressure system leading to the front fluid chamber 139B might fail at the same time.